In conventional practice, supermarkets and convenient stores are equipped with display cases, which may be open or provided with doors, for presenting fresh food or beverages products to customers, while maintaining the fresh food and beverages in a refrigerated environment. Typically, cold, moist air is provided to the product display zone of each display case by passing air over the heat exchange surface of an evaporator coil disposed within the display case in a region separate from the product display zone so that the evaporator is out of customer view. A suitable refrigerant, such as for example R-404A refrigerant, is passed through the heat exchange tubes of the evaporator coil. As the refrigerant evaporates within the evaporator coil, heat is absorbed from the air passing over the evaporator so as to lower the temperature of the air.
A refrigeration system is installed in the supermarket and convenient store to provide refrigerant at the proper condition to the evaporator coils of the display cases within the facility. All refrigeration systems comprise at least the following components: a compressor, a condenser, at least one evaporator associated with a display case, a thermostatic expansion valve, and appropriate refrigerant lines connecting these devices in a closed circulation circuit. The thermostatic expansion valve is disposed in the refrigerant line upstream with respect to refrigerant flow of the inlet to the evaporator for expanding liquid refrigerant. The expansion valve functions to meter and expand the liquid refrigerant to a desired lower pressure, selected for the particular refrigerant, prior to entering the evaporator. As a result of this expansion, the temperature of the liquid refrigerant also drops significantly. The low pressure, low temperature liquid evaporates as it absorbs heat in passing through the evaporator tubes from the air passing over the surface of the evaporator. Typically, supermarket and grocery store refrigeration systems include multiple evaporators disposed in multiple display cases, an assembly of a plurality of compressors, termed a compressor rack, and one or more condensers.
Additionally, in certain refrigeration systems, an evaporator pressure regulator (EPR) valve is disposed in the refrigerant line at the outlet of the evaporator. The EPR valve functions to maintain the pressure within the evaporator above a predetermined pressure set point for the particular refrigerant being used. In refrigeration systems used to chill water, it is known to set the EPR valve so as to maintain the refrigerant within the evaporator above the freezing point of water. For example, in a water chilling refrigeration system using R-12 as refrigerant, the EPR valve may be set at a pressure set point of 32 psig (pounds per square inch, gage) which equates to a refrigerant temperature of 34 degrees F.
As in conventional practice, evaporators in refrigerated food display systems generally operate with refrigerant temperatures below the frost point of water, frost will form on the evaporators during operation as moisture in the cooling air passing over the evaporator surface comes in contact with the evaporator surface. As frost builds up on the evaporator surface, the performance of the evaporator deteriorates and the free flow of air through the evaporator becomes restricted and in extreme cases halted. Consequently, it is customary to equip a refrigerated food display system with a defrost system which may be selectively or automatically operated, typically one to four times in a 24-hour period for up to one hundred and ten minutes each cycle, to remove the frost formation from the evaporator surface.
Conventional methods for defrosting evaporators on refrigerated food display systems include passing air over an electric heating element and thence over the evaporator, passing ambient temperature store air over the evaporator, and passing hot refrigerant gas through the refrigerant lines through the evaporator. The latter method, commonly referred to as hot gas defrost, hot gaseous refrigerant from the compressor passes in reverse direction through the evaporator. The hot gaseous refrigerant condenses in the frosted evaporator and returns as condensed liquid to an accumulator, rather than directly to the compressor to prevent compressor flooding and possible damage. The latent heat given off by the condensing hot gaseous refrigerant melts the frost off the evaporator.
Although effective to remove the frost and thereby reestablishing proper air flow evaporator operating conditions, defrosting the evaporator has drawbacks. As the cooling cycle must be interrupted during the defrost period, the product temperature rises during the defrost. Thus, product in the display merchandiser may be repeatedly subject to alternate periods of cooling and warming. Also, additional controls must be provided on the refrigeration system to properly sequence defrosting cycles, particularly in stores having multiple refrigerated merchandisers to ensure that all merchandisers are not in defrost cycles simultaneously.
According, it would be desirable to operate a refrigerated merchandiser, in particular a medium temperature merchandiser, in a continuous frost-free state without the necessity of employing a defrost cycle. U.S. Pat. No. 3,577,744, Mercer, for example, discloses a method of operating an open refrigerated display case in which the product zone remains frost-free and in which the evaporator coils remain ice-free. In the disclosed method, a small secondary evaporator unit is utilized to dry ambient air for storage under pressure. The cooled, dehydrated air is then metered into the primary cooling air flow and passed in intimate contact with the surfaces in the product zone. As the air in intimate contact with the surfaces is dehydrated, no frost is formed on the surfaces in the product zone.
U.S. Pat. No. 3,681,896, Velkoff, discloses controlling the formation of frost in heat exchangers, such as evaporators, by applying an electrostatic charge to the air-vapor stream and to water introduced into the stream. The charged water droplets induce coalescence of the water vapor in the air and these charged vapor and droplets collect on the surface of oppositely charged plates disposed upstream of the heat exchanger coils. Thus, the cooling air passing over the heat exchanger coils is relatively moisture-free and frost formation on the heat exchanger coils does not occur.
U.S. Pat. No. 4,272,969, Schwitzgebel, discloses a refrigerator for maintaining a high humidity, frost-free environment. An additional throttling element, for example a suction-pressure-regulating valve or a capillary pipe, is installed in the return line between the evaporator outlet and the compressor for throttling the flow to maintain the evaporator surface above 0 degrees Centigrade. Additionally, the evaporator surface is sized far bigger than the evaporator surface used in conventional refrigerators of the same refrigerated volume, preferably twice the size of a conventional evaporator, and possibly ten times the size of a conventional evaporator.